In recent years, InGaN LED is getting popular owing to its excellent performance in blue and green light regions. The solid-state light source application has made the InGaN LED in tremendous importance, wherein it is already used in keypad, back-lighting in cell phone, car lighting, decoration, and many other areas. However, the total output luminance efficiency is still not enough to enter general lighting applications. Regarding LED of high brightness, its output luminance efficiency can be divided into two parts: internal quantum efficiency and external quantum efficiency. The internal quantum efficiency is determined by the ratio of photons generated versus electrons and hole carriers injected. Thanking to the outstanding performance of the recent commercial organic metallic vapor phase epitaxy (OMVPE) equipment, the internal quantum efficiency can almost reach to 100% of the theoretical value. However, in InGaN LED devices, the external quantum efficiency is generally less than 30%. One major reason is that active quantum layers absorb the generated lights, and most of the generated lights are reflected by four edge surfaces and top and bottom surfaces of the chip. That is, light will be reflected totally by the chip's surface when the light incident angle is grater than the total reflection angle of the chip's surface (about 23 degrees from the axis of the surface plane).
Please simultaneously refer to FIG. 1A showing the top view of the conventional nitride LED, and to FIG. 1B showing the side view of the conventional nitride LED. A LED 80 shown in FIG. 1A and FIG. 1B can be formed via the following steps. Firstly, a substrate 10 is provided, wherein the material of the substrate 10 is such as sapphire, GaN, AlN, etc. Then, a semiconductor layer 30 of a first polarity, an active layer 40, a semiconductor layer 50 of a second polarity, and a contact layer 55 are sequentially epitaxially grown on the substrate 10. Afterwards, the aforementioned epitaxial layers are etched, thereby exposing a portion of the semiconductor layer 30 of the first polarity. Then, an electrode 60 of the first polarity and an electrode 70 of the second polarity are deposited respectively on the exposed portion of the semiconductor layer 30 of the first polarity and the contact layer 55 via thermal evaporation, e-beam evaporation, or sputtering, etc.
Such as shown in the top view of FIG. 1A, after the light rays emitted from point d and point e in the active layer (not shown) respectively is totally reflected several times by the boundary, the light is still not emitted out of the LED 80. Besides, such as shown in the side view of FIG. 1B, even though the light rays respectively emitted from point f and point g in the active layer 40 can eventually be emitted out of the LED 80, most of the light rays have been absorbed by all layers of the LED 80 and only few light rays can actually go outside the LED 80 since the light rays have been totally reflected several times. Hence, there is a need to find a solution for the aforementioned problem.